1. Field of the Invention
The present invention relates to X-ray screening systems for airport luggage, cargo, parcels, inter-modal containers, and the like; and, more particularly to screening systems that utilize radiation transmitted through and scattered from an object under inspection to detect weapons, narcotics, explosives or other contraband.
2. Description of the Prior Art
X-ray inspection systems that use transmitted radiation have conventionally been employed to detect the presence and shape of high-Z material (Z refers to atomic number) such as steel. The principal objective of these systems is the detection of weapons, such as guns, knives, bombs and the like. Although transmission systems can be used to detect items with lower Z, in practice they are often considered to be less effective for such items. One approach is provided by dual energy transmission X-ray systems, which have been used to improve the detection of low-Z material. Such systems measure the different attenuation that high and low energy transmitted X-rays inherently experience as a result of passage through a material having some effective atomic number (Zeff). This principle has allowed the identification of virtually any material, so long as the material is not covered by a material with a different Z. In order to overcome the material overlaying problem, it has been proposed that X-ray transmission be effected from different directions using two or more X-ray sources, or that the object be scanned from all sides and the results be evaluated with computed tomography.
Another approach for identifying low-Z material involves detecting both Compton scattered radiation and transmitted radiation. Low-Z materials such as explosives, narcotics, and other organics generate comparatively more scattered radiation than high-Z material like iron. This scattered radiation differential provides a basis for distinguishing between low-Z and high-Z material in instances where the low-Z material is concealed behind high-Z material. Systems may beneficially detect forward or backward scattered Compton radiation or both. Known systems that combine detection of transmitted and Compton scattered radiation include those disclosed by U.S. Pat. No. RE 28,544 to Stein et al., U.S. Pat. No. 5,313,511 to Annis et al., U.S. Pat. No. 6,661,867 to Mario et al., and U.S. Pat. No. 7,072,440 to Mario et al. All of these patents are incorporated herein in the entirety by reference thereto.
Two physical processes principally govern the atomic-level interaction between X-rays and material objects on which common parcel scanning systems are based, namely photoelectric absorption (sometimes termed photo absorption) and Compton scattering. Depending on the radiological characteristics of the objects in a parcel and the energy of the incident X-ray beam, different fractions of the X-ray photons either pass through the parcel object without any interaction, or interact via photoelectric absorption or Compton scattering. In the former, an incident photon is fully absorbed, with transfer of all its energy to an atomic electron. On the other hand, Compton scattering causes the incoming photon to lose some fraction of its energy and to be re-emitted, or scattered, in a direction away from the incident direction, generally defined by an angle θ measured from the incident direction (θ=0°). By convention, Compton scattering for which 0°<θ<90° is termed forward scattering, whereas backward scattering is characterized by values of 90°<θ<180°. Radiation at θ≅90° is called side scattering.
Among the more troublesome problems with systems that use both X-ray transmission and Compton scatter to create images are poor resolution and high noise content. Some of the causes of these problems can be traced to: a) the relatively poor light collection methods used in converting X-ray photons to light photons; and b) photon integration. Detectors using relatively slow phosphors oftentimes create undesirably blurred images owing to the slow response time of the excited phosphor. Although photon integration used in conventional signal generation and processing affords advantages at high X-ray rates, it can result in noisy images, particularly in cases where the transmitted or scattered X-ray rates are relatively small. For example, U.S. Pat. No. 5,260,982 to Fujii et al. discloses a scattered radiation imaging apparatus. The Fujii et al. apparatus employs long persistence phosphor type X-ray detectors and photon integration, resulting in relatively low resolution.
A further difficulty arises from the wide dynamic range of X-ray intensity that practical scanning systems must accommodate. The impact of this wide range is especially challenging as a scanned object comprising portions with very different radiological character (i.e., photoelectric absorption and Compton scattering behavior) passes through the inspection zone. For example, a parcel might contain a metal weapon and an explosive device comprising organic explosive material and a detonator connected to electronic circuitry. These contraband objects are composed of materials having very different average atomic number and density, and thus very different radiological properties. As the different portions pass through the X-ray beam, the X-ray flux at the transmission and scatter detectors changes very rapidly. In the transmission channel, a high flux impinges on the detector when nothing is present in the inspection zone, but that intensity can drop by several orders of magnitude as a radiologically dense item, such as a massive metallic object, passes into the zone. Similar effects are seen in backscattering. Ideally, a practical detection system would accommodate these rapid changes in both channels and be capable of accurately determining the full range of X-ray intensities to give reliable imaging.
Thus, there remains a need in the art for a system and a method of X-ray inspection that would afford more rapid scanning and improved image quality, including higher resolution, reduced noise, and sharper definition of the image. Such improvements would permit items of interest, such as bombs, firearms and other weapons, currency, narcotics, incendiary materials, agents that pose biological, chemical, or radiological danger to people or property, and other contraband shapes and accompaniments to be recognized. Better systems are highly sought, especially in the context of customs and airport screening, but would be equally valuable for courthouses, stadiums, schools, government offices, nuclear power plants, military installations, correctional institutions, border control, and other public venues that might be targets of terrorist or similar criminal activity, and for inspection of cargo being shipped by any mode of conveyance.